Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, throttling losses, in-cylinder heat losses, as well as nitrogen oxide (NOx) emissions. As a result, fuel economy is improved, at part throttle loads and at higher load levels such as during engine boost. As an example, by recirculating a portion of the engine's exhaust back to the engine cylinders, the oxygen in the incoming air stream is diluted and gases inert to combustion act as absorbents of combustion heat to reduce peak in-cylinder temperatures. Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reduces the amount of NOx generated from combustion. Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to other engine cylinders. Therein, all of the exhaust from the dedicated cylinder group is recirculated to the intake manifold. As such, this allows a substantially fixed amount of EGR to be provided to engine cylinders at most operating conditions. By adjusting the fueling of the dedicated EGR (DEGR) cylinder group (e.g., to run rich), the EGR composition can be varied to include species such as Hydrogen and CO which improve the EGR tolerance of the engine, resulting in fuel economy benefits.
When one or more cylinders are dedicated to providing EGR, under standard fueling and controls, the EGR fraction in the charge flow is simply the ratio of the number of EGR cylinders to the total number of cylinders. As an example, an engine comprising one DEGR cylinder out of a total of four cylinders will operate at 25% EGR if all cylinders are operated similarly. While such an arrangement simplifies engine operation in terms of controls, hardware devices, etc., the simplified operation results in a general lack of control over the system. For example, a key disadvantage is the inability to reduce EGR rate at light loads, where combustion stability is a constraint. Another example where lack of control may be disadvantageous is during transient conditions where the pressure of the charge flow in the intake manifold can change more rapidly than the pressure of the exhaust in the exhaust manifold of the dedicated EGR cylinder(s), such as when the driver tips out of the pedal causing the throttle to close quickly. In such an example, the EGR fraction provided may increase significantly over the expected or desired EGR fraction. Deviations from expected or desired EGR fractions may lead to undesired operating conditions, such as cylinder misfire, and combustion instability. As such, it is desirable to enable control over dedicated EGR during light loads and transient conditions, without substantially increasing costs.
US Patent Application US 2015/0369180 teaches that during conditions where an engine shutdown is imminent, requested, or initiated, fueling to the DEGR cylinder(s) may be stopped prior to deactivating the non-DEGR cylinders and shutting down the engine. Furthermore, during engine starting conditions, fuel and spark may be supplied to non-DEGR cylinders, where activation of the DEGR cylinder(s) is initiated responsive to the engine reaching a stable speed. However, the inventors herein have recognized potential issues with such a method. For example, stopping fueling to the DEGR cylinder while maintaining fueling of the non-DEGR cylinders may result in an increase in noise, vibration, and harshness (NVH) constraints under some conditions of vehicle operation due to the uneven firing intervals in the remaining non-DEGR cylinders. Further, in some examples the vehicle may be continuously operated at an engine speed or load where continued engine operation with DEGR may result in combustion stability issues, without proceeding to an engine off state. In such an example, operating the vehicle continuously with non-DEGR cylinders may negatively impact fuel economy, in addition to increasing noise, vibration, and harshness of the operating conditions. Furthermore, operating a vehicle without EGR may in some examples lead to increased NOx emissions.
US Patent Application US 2015/0136074 teaches methods for expediting purging of EGR in a hybrid vehicle during decreasing engine load conditions, where due to a large transport delay between the LP-EGR valve and the combustion chamber, EGR may not be decreased as fast as required. The delay in purging may lead to combustion stability risks. As such, if the vehicle system battery is unable to accept further charge, US 2015/0136074 teaches disabling engine fueling responsive to decreasing engine load conditions, and propelling the vehicle via battery power. The engine may be spun unfueled via the motor while maintaining open the LP-EGR valve and an intake throttle, to purge the EGR system and intake air induction system of exhaust residuals. In an alternate example, if the battery is deemed able to accept further charge, EGR purging may be enabled by operating the vehicle in a generating mode, by closing the EGR valve and operating the engine with output torque greater than demanded torque, and charging the system battery. With the EGR valve closed and the engine in operation, EGR may be quickly purged from the intake. However the inventors have recognized potential issues with such a method. For example, US2015/0136074 does not teach methods for operating a vehicle wherein one or more cylinders comprise DEGR cylinders. Additionally, US2015/0136074 does not teach methods for maintaining EGR during decreasing load conditions, where NOx emissions may increase if EGR is not able to be maintained during engine-on conditions. As such, the inventors have recognized that alternate methodology may be utilized in order to maximize fuel economy and minimize combustion stability issues, while maintaining low NOx emissions, particularly with regard to low-load events where the system battery is capable of accepting further charge.
Thus, the inventors herein have developed systems and methods to at least partially address the above issues. In one example, the issues described above may be addressed by a method including recirculating exhaust gases from one or more cylinders of an engine to the remaining cylinders without controlling the amount of the recirculated exhaust gases; controlling power output of the engine to a desired power to deliver the driver demanded power at the wheels of the vehicle; and under light loads of the engine, increasing the power beyond the desired power, and recharging an onboard energy storage device to reduce the power to the desired power while maintaining the amount of exhaust gas recirculation. In modes of operation other than responding to the driver demanded power at the wheels, for example during idle speed control, the engine power is controlled to achieve a desired engine idle speed. In another mode of operation when vehicle speed control is active, the engine power is controlled to achieve a desired vehicle speed.
In one example, the method includes stopping fuel injection to the one or more cylinders that recirculate exhaust gas to the remaining cylinders; stopping fuel injection to the remaining engine cylinders; and propelling the vehicle via energy from the onboard energy storage device, when the energy storage device is unable to accept further energy storage. Furthermore, the method comprises ceasing the vehicle propulsion from the energy storage device and resuming fueling the one or more cylinders that recirculate exhaust gas to the remaining cylinders when the charge state of the energy storage device exceeds a predetermined value. In this way, responsive to light loads of the engine, and an indication that the energy storage device is capable of accepting further energy storage, engine operation may be maintained while maintaining the amount of exhaust gas recirculation, thus mitigating potential combustion stability issues associated with continued EGR at light loads, and additionally mitigating increases in NOx emissions by maintaining EGR.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.